Native touch is letterbox-scaled twice when client canvas and video aspect ratios differ

[jttdev]

Describe the bug

Native-touch coordinates that the Moonlight client already normalizes to the
visible video area are transformed through Sunshine's client touch port a
second time. With a 16:9 stream displayed inside a 4:3 client canvas, Linux
Touch passthrough receives full-range X but only the top 75% of Y.

I searched existing issues and read the input documentation. This is related
to, but more specific than #2166: the client coordinates are already correct,
and the additional 0.75 factor follows the client-canvas/video aspect-ratio
mismatch.

Reproduction

Host:

• Ubuntu 26.04, x86_64
• KDE Plasma/Wayland, KMS capture
• NVIDIA RTX A4500 Laptop GPU, driver 595.84
• Sunshine deb 2026.516.143833, commit 14ffa6f
• captured output: 3840x2160 (16:9)

Client:

• iPad running VoidLink Extreme 3.4.5
• 4:3 client canvas
• Native Touch enabled

1. Enable min_log_level = 0 and restart Sunshine.
2. Start the 16:9 stream.
3. Sweep native touch from the top to the bottom of the visible video.
4. Compare Sunshine's touch-packet x/y with
   EV_ABS/ABS_MT_POSITION_X/Y from Touch passthrough.

Observed:

• client X: 0..0.9996; device X spans the full axis.
• client Y: 0..0.9937; device Y is 0.7504 * client_y.
• mapping is top-anchored; there is no lower-letterbox offset.
• local correction: TECHNIKA_TOUCH_CAL=0,0,65535,49484.

The measured bottom point was client y=0.9937, device/game
y=8104/10800; expected video-bottom raw Y was 8155, and the observed
normalized-device anchor was 49484. A full-glass sweep confirmed that the
client itself sent nearly the full normalized Y range.

Expected behavior

Native touch (0,0)..(1,1) is over the visible video area, so the virtual
touchscreen should receive the full selected-display range exactly once.
Client letterbox exclusion should not be reapplied by the host.

Source evidence / latest prerelease

Sunshine passes native touch normalized to 65535 x 65535 through
client_to_touchport():

• measured stable:
  client_to_touchport(),
  native-touch caller
• latest prerelease v2026.726.710 (7cb9207):
  client_to_touchport(),
  native-touch caller

The mapping implementation is equivalent between the measured stable build
and latest prerelease. To be precise, the hardware measurement was made on
stable; I source-verified the latest prerelease on 2026-07-29 but did not
substitute its binary during an unrelated patched-inputtino deployment.